The present disclosure relates to a digitally-controlled oscillator, a frequency synthesizer and a radio communication device for use in a semiconductor integrated circuit.
In order to achieve a reduction in size and an increase in speed of semiconductor integrated circuits, a frequency synthesizer which employs a digitally-controlled oscillator (DCO) circuit for controlling an output frequency using digital values has been studied instead of a voltage-controlled oscillator (VCO), which controls an output frequency using analog voltages.
In the DCO circuit, it is necessary to change a frequency within a wide range, and reduce a minimum frequency change amount to the extent possible while achieving a constant change amount, so as to perform wide-range modulation while reducing quantization noise.
A DCO circuit as shown in FIG. 19 has been proposed so as to change a frequency within a wide range while reducing a minimum frequency change amount (see, for example, Japanese Unexamined Patent Application Publication No. 2002-33660 or U.S. Pat. No. 6,734,741). As shown in FIG. 19, the conventional DCO circuit 200 has an inductor 201 and a variable capacitance unit 202. The inductor 201 and the variable capacitance unit 202 form an LC tank circuit, which generates an oscillating signal having a frequency which depends on the inductance of the inductor 201 and the capacitance value of the variable capacitance unit 202.
The variable capacitance unit 202 has a first variable capacitance unit 205, a second variable capacitance unit 204, and a third variable capacitance unit 203. The first, second and third variable capacitance units 205, 204 and 203 each have a plurality of varactor elements. Each varactor element is a variable-capacitance element having a capacitance value varying between a low value (low capacitance state) and a high value (high capacitance state).
The first, second and third variable capacitance units 205, 204 and 203 include respective varactor elements having different capacitance values and different capacitance change amounts. Therefore, for example, the frequency can be finely changed by changing the capacitance states of the varactor elements included in the first variable capacitance unit 205. The frequency can be largely changed by changing the capacitance states of the varactor elements included in the third variable capacitance unit 203.